Thirty-four years ago I took my first weed science class at Auburn University; AGY 434. I struggled —
mightily. Yet, that class pointed me down the career path and eventually to my current vocation. The instructor was famous for multiple choice exams, with the answer choices ranging from A through M. A through E would be technical choices. F through M would be infuriating statements like A, B, and maybe D but never C. Students are highly vulnerable to intimidation, and I frequently lost sight of the fundamental weed science lessons being evaluated.
One exam in particular dealt with losses that weeds cause. Although the questions were ridiculously nit-picky, the concepts being tested were actually very simple. This is certainly the case with weeds and the losses they cause in food plots. The point to this personal testimony is to encourage growers not to dwell on the "what-ifs" when making weed control decisions and focus on the simple concept that weeds are the great early-season thief of food plot productivity.
EARLY-SEASON VULNERABILITY TO WEEDS
To begin this discussion, consider the stage of food plot development when forages are most susceptible to losses from weeds — the establishment period. All forage species, particularly perennial legumes, are slow-growing as seedlings and vulnerable to weed competition. This vulnerability might be expressed as slower crop development and even stand reduction as weeds compete with forages for sunlight, water and soil nutrients.
Refer to the data presented in Table 1. These trials were conducted in Pennsylvania, with the data presented being part of a larger data set. The abbreviated data set is a simple comparison of early-season alfalfa growth between plots with weedy volunteer oat controlled with sethoxydim (Arrest) and a non-treated control. Weed control meant more alfalfa leaflets, greater survival after seeding (pereniation), and greater overall forage yield compared to alfalfa with no weed control. This equates to more nutritious forage to attract and sustain the deer herd when weeds are controlled early-season.
This phenomenon was also studied by weed scientists with Oregon State University and USDA-ARS (Table 2). Newly seeded alfalfa was maintained weed-free with handweeding for varying periods the first 260 days after establishment. At the end of the 260-day period, above ground weed and alfalfa growth were collected. Obviously, the longer weeds are controlled in seedling alfalfa, the greater the alfalfa yield. Closer study of the data shows that the critical period of early-season weed control for maximum alfalfa yield was about 170 days after seeding — nearly six months. Weed control for 170 days after seeding alfalfa increased yield by 268 percent compared to alfalfa yield with no weed control. Using labor-intensive handweeding weed control in food plots is not practical and that is why we use selective herbicides like Slay and Arrest. Regardless of how weeds are controlled, this data is irrefutable documentation of the importance of early-season weed control.
All of this makes sense and is completely intuitive. Early-season weed control is absolutely essential to forage crop establishment, particularly when conditions are austere. While these studies were conducted
with alfalfa across a wide geographical range, the general relationship is fundamental for any forage crop and location.
Similar studies were recently conducted in Pennsylvania and those studies produced generally comparable results. However, the recent trials had an additional variable — the effect of baseline weed populations on the importance of early-season weed control. Weed control in sites with heavy weed infestations needed to begin immediately after seeding alfalfa, while weed control at sites with lower weed infestations could be
delayed a few weeks.
This has direct implications on how we choose food plot sites and the intensity of weed control. In cases where food plot managers know or strongly suspect serious weed problems, weed control must be proactive and aggressive, beginning even before the forages are seeded. Pre-plant or site-preparation weed control is a recommended strategy to lower baseline weed densities before seeding the forage. This includes frequent stale seedbed (pre-plant) tillage and fallow applications of glyphosate to kill emerged weeds. This will be further discussed in a future article.
A practical consideration that further validates the importance of early-season weed control is the opportunity to control small weeds at a stage of growth when they are vulnerable to herbicides. Selective herbicides such as Arrest and particularly Slay are far more effective on small (seedling) weeds than large weeds. Not only is early-season weed control important to prevent forage yield losses, selective herbicides perform better by targeting seedling weeds compared to later applications to larger weeds.
MAXIMIZING FORAGE COMPETITION WITH WEEDS
So far, this discussion has focused solely on how early-season weeds decimate seedling forages. Forage crops that are adapted to the region, properly managed, and growing under good conditions are capable of competing on near-equal terms with weeds. Outside of unpredictable growing conditions, the practices used to prepare the seedbed and sow the forage directly affect the uniformity and quality of the forage stand. Voids or skips in the forage stand promote weed infestation. After all, weeds are opportunists. My full-time job as a weed scientist includes research and outreach programs to serve organic growers and weed control is their biggest production challenge. In organic crop production, there are few corrective weed control options and cultural practices are a widely used preventative weed control tool. In that sense, weed control in food plots is conceptually the same as in organic crop production. Simply stated, let the crop’s innate competitive ability do the ‘heavy lifting’ by suppressing early season weed growth. This benefit is captured by using sound forage crop production practices that ensure an optimum and uniform crop stand.
To summarize these points, I could not resist using this tool — a multiple choice question:
What best describes the most effective technique to manage early-season weeds and minimize losses in forages planted in food plots?
A. Choose a forage species that is adapted to the region.
B. Use pre-plant tillage and herbicides to reduce the baseline
weed density.
C. Uniformly sow the forage at the recommended seeding rate.
D. Use an appropriate selective herbicide to control small weeds
in seedling forages.
E. All of the above.
Table 1. Effect of weed control with sethoxydim on alfalfa growth and yield, eight months after
seeding in Pennsylvania1.
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Shoot length (in.)
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Leaflets (no./plant)
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Perenniated (%)
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Alfalfa foliage yield (lbs./A)
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Sethoxydim (Arrest®)2
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3.7
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10.9
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65
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2390
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No weed control
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4.0
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8.5
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62
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390
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1Stout, W. L., R. A. Byers, K. T. Leath, C. C. Bahler, and L.
D. Hoffman. 1992. Effects of weed and invertebrate control on alfalfa
establish-ment in oat stubble. J. Prod. Agric. 5:349-352. 2Herbicide applied 19 days after seeding alfalfa to control
volunteer oat.
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Table 2. Effect of duration of weed control on
seedling alfalfa growth, Prosser, WA1.
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Duration of weed control2 (days after seeding)
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Above-ground dry matter nine months
after seeding (lbs./A)
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Weeds3
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Alfalfa
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no weed control
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6440
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1380
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8
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5730
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2160
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15
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4760
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2420
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22
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3090
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3290
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36
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2400
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3340
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65
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960
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4330
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170
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420
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5080
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193
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116
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4840
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214
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44
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5130
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235
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18
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4560
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260 (full season weed control)
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0
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4720
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1Fischer, A. J., J. H. Dawson, and A. P. Appleby. 1988.
Interference of annual weeds in seedling alfalfa (Medicago sativa). Weed Sci.
36:583-588. 2Alfalfa seeded in August and weeds
controlled with handweeding for a maximum of 260 days. 3Weeds were a composite of a cool season annual grass (downy
brome) and cool season broadleaf weed (tumble mustard). The combined weed
density averaged 44 weeds/ft2 .
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Dillehay, B. L., W. S. Curran, and D. A.
Mortensen. 2011. Critical pe-riod for weed control in alfalfa. Weed Sci.
59:68-75. W
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